1. Field of the Invention
The present invention relates to a frequency converter circuit for converting the frequency of a carrier chrominance signal into the frequency of an NTSC signal in a process of converting a Hi-Vision signal into a NTSC signal.
2. Background Art
In the NTSC method, in order to maintain compatibility between an existing color television method and a monochrome television method, output signals for the three primary colors of red (R), green (G) and blue (B) are converted into a luminance signal Y and two color signals (color difference signal or chrominance signal) by a converter circuit called a matrix circuit. The luminance signal is transmitted in the same manner as in a monochrome television method, with the color signals being transmitted by being interleaved in the interval between the luminance signal spectra using multiplexing, so that transmission is accomplished with a 6 MHz band width per channel.
With the help of analog technology, two color signals are multiplexed with a luminance signal in the following manner: the two color signals are separately modulated in amplitude using two color subcarrier signals which are similar in frequency but different by 90 degrees in phase, and the resultant carrier chrominance signals given by the modulated signals being added together are inserted in the interval between the sidebands of the luminance signal.
With the help of digital technology, a carrier chrominance signal is generated in the manner as shown in FIG. 3. From color difference signals (R-Y) and (B-Y) each consisting of an eight bit digital signal, the color difference signals (R-Y), (B-Y) and their complement or sign-inverted color difference signals -(R-Y), -(B-Y) derived by passage through sign inverter circuits Ia and Ib, are input, respectively, to the first second, third and fourth input terminals T1, T2, T3 and T4, and are switched in sequence by means of a clock signal CK with a frequency four times higher than that of the color subcarrier of a frequency fsc.
In this way, the color difference signals (R-Y), (B-Y), -(R-Y) and -(B-Y) are switched in sequence as shown in a waveform chart of FIG. 4 by the clock signal CK (FIG. 4(a)), and appear alternately in a period of 1/(4 fsc) at the output terminal T5 of a selector S, to be output therefrom, as a carrier chrominance signal (FIG. 4(b)).
This corresponds to the process in which the components of color subcarriers fsc(R-Y) and fsc(B-Y) (different by 90 degrees from each other) are extracted by a carrier chrominance signal being sampled by means of a clock signal with a frequency of 4 fsc.
Conversion of a high definition television method, for example, the Hi-Vision method into the NTSC method, involves conversions of the number of scanning lines, aspect ratio, and field frequency. According to the BTAS-001 studio standard of the Hi-Vision method, the field frequency fv(H) is 60.00 Hz, while the field frequency for the NTSC method is 59.94 Hz. Thus, the attempt to convert a Hi-Vision signal into an NTSC signal without converting the field frequency results in a color subcarrier frequency fsc(H) of 3.5831 MHz, which is slightly, that is, 1001/1000 times, higher than the color subcarrier frequency fsc(N) (=3.5795 MHz) for the NTSC method.
For this reason, if color difference signals (R-Y), (B-Y) converted from RGB signals of Hi-Vision whose field frequency is 60.00 Hz are switched alternately with a clock signal CK of the frequency 4 fsc(H) and the resultant carrier chrominance signals are sent to a NTSC receiver, proper color demodulation cannot be, or is disadvantageously, performed. Neither can color restoration be properly performed because the color signal is demodulated with the color subcarrier of the frequency fsc(N) in a NTSC receiver.